virnetdevtap: Don't crash on !ifname in virNetDevTapInterfaceStats

virnetdevtap: Don't crash on !ifname in virNetDevTapInterfaceStats

https://bugzilla.redhat.com/show_bug.cgi?id=1595184

Some domain <interfaces/> do not have a name (because they are
not TAP devices). Therefore, if
virNetDevTapInterfaceStats(net->ifname, ...) is called an instant
crash occurs. In Linux version of the function strlen() is called
over the name and in BSD version STREQ() is called.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>